Bar code scanners are widely used in numerous applications in which bar codes are employed to track goods and to track and facilitate transactions involving goods. Speed and accuracy in scanning is always highly desirable, and producing a scan pattern that provides the greatest possible coverage contributes significantly to speed and accuracy. One significant contributor to the coverage provided by a scan pattern is the number of scan lines comprising the scan pattern. For example, a scan beam is produced when a laser beam strikes a rotating polygonal spinner and is directed to a scan window, typically by a series of fixed mirrors. The scan beam emerges from a scanner window. The rotation of the spinner causes the scan beam to sweep out a scan line. As the spinner rotates, the laser beam is deflected by succeeding facets of the polygon, producing a plurality of scan lines to form a scan pattern.
One major limitation constraining the number of scan lines in a scan pattern is the number of facets that the polygonal spinner may have. Most prior art scanners use spinners whose facets are as flat as possible in order to reduce distortion in the laser spot and to maximize the efficiency of collection of light that is directed back into the scanner as a result of a reflection created when a scan pattern strikes a bar code. If the facets of the spinner are flat, adding additional flat facets to the spinner reduces the angles between adjacent facets and reduces the angles at which different scan lines are oriented with respect to one another, thereby limiting the coverage produced by the scan pattern. A smaller angle between scan lines leads to a higher tendency toward overlap of the area covered by the scan lines.
A spinner may be designed to have concave facets, allowing for an increased number of facets and also for a relatively great angle between facets. However, the use of a spinner having concave facets creates a number of problems. One problem is that the speed at which the scan beam sweeps out a scan line, and, therefore, the speed at which the scan beam travels across an object being scanned, will vary significantly if the spinner rotates at a fixed speed, causing timing variations in the timing of light reflected from a bar code. The other problem is that the intensity of light on an object being scanned, and therefore the intensity of light reflected back into the scanner, decreases when the laser beam striking the spinner is near an edge separating two facets of the spinner. If accommodations are not made for such variations in timing and light intensity, the variations may interfere with accurate scanning.
There exists, therefore, a need for a scanner that can produce a scan pattern with a relatively large number of scan lines oriented at relatively large angles with respect to one another, and which can adapt to any variations in light and pattern characteristics caused by features of a reflective spinner used by the scanner to create the scan pattern.